The present invention relates to a transfer sheet suitable for forming high-precision patterns for layers such as electrode layers, dielectric layers and barrier layers in plasma display panels (hereinafter referred to as "PDPs"), field emission displays (FEDs), liquid crystal display devices (LCDs), vacuum fluorescent displays, hybrid integrated circuits, etc. The present invention also relates to a pattern-forming method using the transfer sheet.
As well known so far in the art, patterns of the type described above are formed by coating an ink for conductors or insulators in the form of a desired pattern on a substrate, e.g. a glass or ceramic substrate, by screen printing, and then carrying out firing, thereby forming a thick pattern in close contact with the substrate. According to this method, for instance, fine lines having a line width of 100 micrometers and a height of 100 micrometers are formed by repeating superposition printing a plurality of times.
However, the method of forming a pattern by repeating screen printing a multiplicity of times has several problems.
First, expansion and contraction of the screen used for printing are unavoidable, and various patterns are superimposed one upon another in most of the actual printing processes for forming patterns. Therefore, misalignment between patterns is likely to occur.
Second, because a screen is used as a printing plate, pattern distortion is likely to occur, and it is difficult to form micro patterns.
Third, wiping is necessary for each printing because of the migration of the pattern-forming material to the back of the screen plate; therefore, automation is difficult.
Fourth, a pattern size achievable by screen printing is at most about 100 micrometers for width, and pattern shape is about 0.5 as expressed by a half width-to-bottom width ratio (the half width is the width of a pattern-forming layer at a position half the height thereof). For a barrier layer in a PDP which should be coated to an as-dried thickness of from about 150 micrometers to about 200 micrometers, for instance, it is necessary to increase its bottom area, and hence it is impossible to form any fine pattern.
Fifth, for a multilayer pattern, layers are stacked one upon another while alignment is carried out per layer because it cannot be formed in one operation. In this regard, there is a problem that it is difficult to improve alignment accuracy.
Sixth, it is impossible to form a thick pattern having a high aspect ratio because the ink tails due to its fluidity. Furthermore, it is difficult to effect condition controls for preventing contamination with foreign matters or the like because operations must be carried out in an open system, and a great deal of time is needed for pattern fabrication in the present state of art.
There is also known a so-called sand blasting technique wherein a pattern-forming layer is solid coated on a substrate by repeating screen printing a multiplicity of times, then forming a sand blasting mask on the pattern-forming layer by using a photosensitive resist, and finally jetting an abrasive to perform patterning of the pattern-forming layer ("Electronic Materials", 1983, No. 1, p. 138). The use of this subtractive processing method enables a barrier layer to be formed into a desired shape in which the wall surface steeply rises vertically, and the barrier layer is narrow in width and high in height. Adoption of the photolithography technique for patterning of the resist enables the pattern accuracy to increase and also permits the panel size to increase.
Recently, a barrier layer forming method that uses a transfer sheet having a glass paste provided on a base film has been proposed [Japanese Patent Application Unexamined Publication (KOKAI) No. 8-273536]. This method uses a transfer sheet provided with a glass paste layer and is advantageous in that the process of producing barriers for a PDP panel can be simplified. However, it has been found that in the proposed method a volatile substance in a barrier-forming layer is removed after a resist pattern has been formed on the top surface of the transferred barrier-forming layer as the second step in the process; therefore, the resist pattern is likely to separate owing to bleeding of the volatile substance, and this may cause a problem in the shape of the thick pattern formed.
A first object of the present invention is to provide a transfer sheet suitable for forming micro patterns for electrodes, resisters, barriers, etc. in image display devices such as PDPs and liquid crystal displays, thermal heads, integrated circuits, and so forth.
A second object of the present invention is to provide a pattern-forming method of forming a pattern having superior surface smoothness, a uniform thickness and high profile precision in a reduced period of time and in high yields.
A third object of the present invention is to improve the conventional thick pattern-forming method using the sand blasting technique and to provide a thick pattern-forming method capable of forming a thick pattern in a desired shape with high accuracy.
A fourth object of the present invention is to provide a thick pattern-forming method capable of forming a thick pattern without separation or cracking of a thick pattern-forming layer when the transfer sheet is bent or taken up.
A fifth object of the present invention is to provide a pattern-forming method suitable for transfer-forming a PDP constituent layer, particularly an electrode-forming layer, in a predetermined pattern shape on a PDP glass substrate.
A sixth object of the present invention is to provide a pattern-forming method which enables a PDP constituent layer, particularly an electrode-forming layer, to be formed in a pattern shape on a PDP glass substrate at low costs and in high yields in a reduced period of time, and which also makes it possible to form a laminated structure having superior surface smoothness, a uniform thickness and high profile precision.
A seventh object of the present invention is to provide a pattern-forming method capable of forming a dielectric layer over a substrate, including a raised electrode pattern, without a transfer failure in the fabrication of a PDP or a multilayer electrode panel, and thus providing high yields.